Regenerable MgO Promoted Metal Oxide Oxygen Carriers for Chemical Looping Combustion

ABSTRACT

The disclosure provides an oxygen carrier comprised of a plurality of metal oxide particles in contact with a plurality of MgO promoter particles. The MgO promoter particles increase the reaction rate and oxygen utilization of the metal oxide when contacting with a gaseous hydrocarbon at a temperature greater than about 725° C. The promoted oxide solid is generally comprised of less than about 25 wt. % MgO, and may be prepared by physical mixing, incipient wetness impregnation, or other methods known in the art. The oxygen carrier exhibits a crystalline structure of the metal oxide and a crystalline structure of MgO under XRD crystallography, and retains these crystalline structures over subsequent redox cycles. In an embodiment, the metal oxide is Fe 2 O 3 , and the gaseous hydrocarbon is comprised of methane.

GOVERNMENT INTERESTS

The United States Government has rights in this invention pursuant tothe employer-employee relationship of the Government to the inventors asU.S. Department of Energy employees and site-support contractors at theNational Energy Technology Laboratory.

FIELD OF THE INVENTION

The disclosure provides an oxygen carrier for a chemical loopingcombustion process comprised of a plurality of metal oxide particles incontact with a plurality of MgO promoter particles. The oxygen carrierexhibits a crystalline structure of the metal oxide and a crystallinestructure of MgO under XRD crystallography over numerous redox cycles.The MgO promoter particles increase the reaction rate and oxygenutilization of the metal oxide when contacting with a gaseoushydrocarbon at a temperature greater than about 725° C. In anembodiment, the metal oxide is Fe₂O₃ and the gaseous hydrocarbon iscomprised of methane.

BACKGROUND

Applications are known where metal-oxide oxygen carriers are utilizedfor the delivery of oxygen via reduction of the oxygen carrier. One suchapplication which has been investigated extensively is chemical loopingcombustion. Chemical looping combustion systems generally utilize a fuelreactor, an air reactor, and a metal oxide oxygen carrier undergoingreduction in the fuel reactor and oxidation in the air reactor. Thereduction in the fuel reactor is facilitated by close contact between afuel and the oxygen carrier. The subsequent oxidation of the carrier inthe air reactor is an exothermic process and a stream of N₂ is exhaustedfrom the air reactor and carries the heat of oxidation to an attachedpower generation island.

Chemical looping combustion cycles provide potentially significantadvantages, enhanced reversibility of the two redox reactions offersimproved efficiencies over traditional single stage combustions, wherethe release of a fuel's energy occurs in a highly irreversible manner.Further, with appropriate oxygen carriers, both redox reactions canoccur at relatively low temperatures, allowing a power station to moreclosely approach an ideal work output without exposing components toexcessive working temperatures. Additionally, and significantly,chemical looping combustion can serve as an effective carbon capturetechnique. Of the two flue gas streams generated, one is comprised ofatmospheric N₂ and residual O₂, but sensibly free of CO₂, while thesecond stream is comprised of CO₂ and H₂O, and contains almost all ofthe CO₂ generated by the system. It is relatively uncomplicated toremove the water vapor, leading to a stream of almost pure CO₂. Forthese reasons, chemical looping combustion systems have been extensivelyinvestigated.

It is understood that high reactivity and oxygen utilization of theoxygen carrier is desired in chemical looping combustion systems inorder to limit the solid inventories utilized in the various processes.Generally, the amount of the bed material in each reactor and the solidcirculation rates between reactors mainly depends on the oxygen carryingcapacity of the carriers. As a result, an important characteristic of asuccessful oxygen carrier is its reactivity in both reduction andoxidation cycles. To increase, reactivity, oxygen carrier particles areoften prepared by depositing a metal oxide phase on an inert supportsuch as SiO₂, TiO₂, ZrO₂, Al₂O₃, YSZ, bentonite, and others, in order tostabilize the metal loading and increase exposed surface area overrepeated reduction-oxidation cycles.

Magnesium oxide (MgO) has additionally been utilized to foster thesupport of certain metal oxides such as Fe₂O. This has generally beenconducted by sintering a Fe₂O₃/MgO mixture at temperatures exceedingabout 1000° C. and stabilizing the oxide by generating MgFe₂O₄. Seee.g., Jin et al., “Development of a Novel Chemical-Looping Combustion:Synthesis of a Solid Looping Material of NiO/NiAl₂O₄ .” Ind. Eng. Chem.Res. 38 (1999). Similarly magnesium has been utilized as a component invarious supporting spinels. See e.g., Ryden et al. “Fe₂O₃ on Ce-, Ca- orMg-Stabilized ZrO2 as Oxygen Carrier for Chemical-Looping CombustionUsing NiO as Additive,” AIChE Journal, Vol. 56, No. 8 (2010), and seeJohansson et al., “Investigation of Fe₂O₃ with MgAl₂O₄ forChemical-Looping Combustion,” Ind. Eng. Chem. Res. 43 (2004), and seeAdanez et al., “Progress in Chemical-Looping Combustion and Reformingtechnologies,” Prog. Energ. Combust. 38 (2012). Generally speaking, inthese oxygen carriers, MgO has been absent as a discrete component. Incases where MgO has been identified within the oxygen carrier, reductiontemperature have been limited to a maximum of 700° C., and very slow orno reaction has been reported. See Jin et al., and see Adanez et al. MgOhas additionally been utilized as a discrete component in conjunctionwith Fe₂O₃ and MnO₂ in order to enhance fluidization, however theresulting mixture exhibits CO selectivity on the order of 90%. See U.S.Pat. No. 2,607,699 to Corner et al., issued Aug. 19, 1952.

Disclosed here is an oxygen carrier comprised of a metal oxide and MgOpromoter particles for the chemical looping combustion of a gaseoushydrocarbon at temperatures greater than about 725° C. The oxygencarrier maintains MgO as a discrete component over numerous redoxcycles, and demonstrably improves the percentage combustion and oxygenutilization of the metal oxide. The MgO component does not function as asupport material for increasing the surface area of the oxygen carrier.Additionally, the oxygen carrier generates CO₂ and H₂O combustion gaseswith a substantial absence of CO and H₂. The effect is temperaturedependent and is generally not observed at temperatures below about 700°C. In a particular embodiment, the metal oxide is Fe₂O₃, and MgOcomprises between 5 weight % (wt. %) and 25 wt. % of the Fe₂O₃/MgOmixture.

The objects, aspects, and advantages of the present disclosure willbecome better understood with reference to the accompanying descriptionand claims.

SUMMARY

The disclosure provides an oxygen carrier comprised of a promoted oxidesolid, where the promoted oxide solid is comprised of a plurality ofmetal oxide particles in contact with a plurality of MgO promoterparticles. The MgO promoter particles increase the reaction rate andoxygen utilization of the metal oxide when contacting with a gaseoushydrocarbon at a temperature greater than about 725° C. In anembodiment, the metal oxide is Fe₂O₃ and the gaseous hydrocarbon iscomprised of methane.

The oxygen carrier may be utilized within a chemical combustion systemhaving a fuel reactor and an oxidation reactor. The fuel reactorreceives a flow of a gaseous hydrocarbon fuel and facilitates contactbetween the gaseous hydrocarbon fuel and the oxygen carrier at anappropriate temperature condition, reducing the metal oxide andgenerating CO₂ and H₂O. The reduced carrier may subsequently enter theoxidation reactor for subsequent oxidation. Following the oxidation, theregenerated metal oxide particles and the MgO promoter particlescomprise a regenerated promoted oxide solid, which may be subsequentlytransported to the fuel reactor for use as the oxygen carrier in acyclic operation. Within the cycle, the MgO promoter particles increasethe reaction rate and oxygen utilization of the metal oxide, butotherwise act as an inert material with no significant oxygen releasecapability. The surface areas of MgO promoted carriers are generally lowindicating that MgO does not act as a support which usually increasesthe surface area. Additionally, the promoted oxide solid exhibits acrystalline structure of both the metal oxide and MgO under XRDcrystallography following the reduction-oxidation cycles.

The MgO promoter particles significantly increase the oxygen transfer ofthe metal oxide and greatly improve the reaction rate and percent ofcombustion of the gaseous hydrocarbon, as compared to the metal oxideacting alone. The addition of an inert material such as MgO can alsoincrease the physical strength of the oxygen carrier. Thus, the additionof MgO promoter particles increases the methane reactivity and physicalstability of the oxygen carrier and has a potential for use improvingindustrial oxygen carriers for the chemical looping combustion process.

Embodiments of the oxygen carrier disclosed are further demonstrated anddescribed in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a chemical looping combustion system.

FIG. 2 illustrates XRD crystallography of a particular embodiment of theoxygen carrier.

FIG. 3 illustrates moles CO₂ generated by a first embodiment of theoxygen carrier for various MgO loadings.

FIG. 4 illustrates moles CO₂ generated by a second embodiment of theoxygen carrier for various MgO loadings.

FIG. 5 illustrates % oxygen used for several embodiments of the oxygencarrier.

FIG. 6 illustrates the effect of temperature on a particular embodimentof the oxygen carrier.

DETAILED DESCRIPTION

The following description is provided to enable any person skilled inthe art to use the invention and sets forth the best mode contemplatedby the inventor for carrying out the invention. Various modifications,however, will remain readily apparent to those skilled in the art, sincethe principles of the present invention are defined herein specificallyto provide an oxygen carrier exhibiting improved performance through theuse of MgO promoter particles.

The disclosure provides an oxygen carrier comprised of a promoted oxidesolid, where the promoted oxide solid is comprised of a plurality ofmetal oxide particles in contact with a plurality of MgO promoterparticles. The MgO promoter particles increase the reaction rate andoxygen utilization of the metal oxide when contacting with a gaseoushydrocarbon at a temperature greater than about 725° C., and may beprepared by physical mixing, incipient wetness impregnation, or othermethods known in the art for generating a plurality of metal oxideparticles in contact with a plurality of MgO particles. The oxygencarrier exhibits a crystalline structure of the metal oxide and acrystalline structure of MgO under XRD crystallography, and retainsthese crystalline structures over subsequent redox cycles. The oxygencarrier may be utilized for the delivery of oxygen to a fuel reactor ina chemical looping combustion process, followed by subsequent oxidationof the reduced metal oxide using an oxidizing gas. In an embodiment, themetal oxide is Fe₂O₃ and the gaseous hydrocarbon is comprised ofmethane.

A chemical looping combustion system within which the oxygen carrierdisclosed here may be utilized is illustrated at FIG. 1. FIG. 1illustrates a chemical combustion system generally at 100 and includesfuel reactor 101. Fuel reactor 101 receives a flow of a gaseoushydrocarbon fuel at 102 and the oxygen carrier at 103, and facilitatescontact between the gaseous hydrocarbon fuel and the oxygen carrier.Fuel reactor 101 further is maintained at a temperature sufficient togenerate a combustion reaction as a result of the contact between thegaseous hydrocarbon and the oxygen carrier, generating CO₂ and H₂O andreducing at least a portion of the oxygen carrier. Following thereduction and oxidation, combustion gases comprised of CO₂ and H₂O witha substantial absence of CO and H₂ exit fuel reactor 101 at exhaust 104,and the reduced carrier exits fuel reactor 101 at 105.

The oxygen carrier entering fuel reactor 101 at 103 is comprised of apromoted oxide solid, where the promoted oxide solid is comprised of aplurality of metal oxide particles and a plurality of MgO promoterparticles, as will be discussed. Under XRD crystallography, the oxygencarrier exhibits a crystalline structure of both the metal oxide andMgO. At least some portion of the plurality of metal oxide particles isin physical contact with the plurality of the MgO promoter particles. Inaddition, the oxygen carrier may be further comprised of an inertsupport in contact with the promoted oxide solid. With reference to FIG.1, within fuel reactor 101, the promoted oxide solid comprising theoxygen carrier may be reduced from a generic MgO-Me_(x)O_(y) compositionto a reduced carrier having generic MgO-Me_(z)O_(y-1) composition, whereMe_(x)O_(y) represents the metal oxide or combination of metal oxidesand Me_(z)O_(y-1) represents reduced metal oxide particles, and whereMgO represents the MgO promoter particles. In an embodiment, the metaloxide is Fe₂O₃, CuO, NiO, Mn₂O₃, Co₃O₄, CaSO₄, MoO or mixtures thereof,and the reduced metal oxide particles are a reduced form of therespective metal oxides.

The reduced carrier exiting fuel reactor 101 at 105 may subsequentlyenter oxidation reactor 106. Oxidation reactor 106 further receives aflow of oxidizing gas such as air, and facilitates contact between thereduced carrier and the oxidizing gas. The temperature of oxidationreactor 106 in conjunction with oxygen contact is sufficient to oxidizethe reduced metal oxide particles comprising the reduced carrier,producing regenerated metal oxide particles, where the regenerated metaloxide particles are oxidations of the reduced metal oxide particles.Following the oxidation, the regenerated metal oxide particles and theMgO promoter particles comprise a regenerated promoted oxide solidhaving the generic MgO-Me_(x)O_(y) composition of the promoted oxidesolid, and the regenerated promoted oxide solid comprises a regeneratedoxygen carrier. The regenerated oxygen carrier may be subsequentlytransported to fuel reactor 101 at 103 for use as the oxygen carrier ina cyclic operation.

Additionally, as is understood, the oxidizing reaction occurring inoxidation reactor 106 may be an exothermic reaction, and heat generatedmay be carried from oxidizing reactor 106 by a gaseous flow exiting at108, and may be sent to and utilized by a power generation cycle.Further, it is understood that FIG. 1 provides an exemplary applicationillustrating a chemical looping combustion process with the oxygencarrier, however this is not intended to be limiting. Within thisdisclosure, it is only necessary that an oxygen carrier comprised of apromoted oxide solid, where the promoted oxide solid is comprised of aplurality of metal oxide particles in contact with a plurality of MgOpromoter particles, be contacted with a heated gaseous hydrocarbon at atemperature greater than about 725° C., producing combustion gases and areduced carrier, where the combustion gases are comprised of CO₂ and H₂Oand where the combustion gases have a substantial absence of CO and H₂,and where the reduced carrier is comprised of a plurality of reducedmetal oxide particles and the plurality of MgO promoter particles, andwhere the reduced metal oxide particles are a reduction product of themetal oxide particles.

Within this disclosure, “metal oxide particles” means particlescomprised of a compound consisting of a transition metal cation boundedwith at least one oxygen atom anion or any material active for CLCreaction. Similarly, “reduced metal oxide particles” means a reducedform of the metal oxides particles, such that “reduced metal oxideparticles” means either: (1) particles comprised of a compoundconsisting of the transition metal cation of the metal oxide particlesbounded with at least one oxygen atom anion, where the ratio of theoxygen atom anion to the transition metal cation in the reduced metaloxide particles is less than the ratio of the oxygen atom anion to thetransition metal cation in the metal oxide particles or (2) particlescomprised of the transition metal comprising the metal oxide particles.Such metal oxide particle—reduced metal oxide particle redox systemscommonly used for the transfer of oxygen are known in the art. See e.g,Adanez et al., “Progress in Chemical-Looping Combustion and Reformingtechnologies,” Prog. Energ. Combust. 38 (2012), among others. In anembodiment, the metal oxide particles are comprised of Fe₂O₃, CuO, NiO,Mn₂O₃, Co₃O₄, CaSO₄, MoO and mixtures thereof, and the respectivereduced metal oxide particles are comprised of Fe₃O₄ and FeO, Cu, Ni,Mn₃O₄, CoO, and mixtures thereof. In a particular embodiment, the metaloxide particles are comprised of Fe₂O₃ and the reduced metal oxideparticles are comprised of Fe₃O₄. In addition, CaSO₄ may also be used inplace of metal oxides.

Within this disclosure, “MgO promoter particles” means particlescomprised of MgO, for example, magnesia or dolomite. In an embodiment,MgO promoter particles are present in the promoted oxide solid such thatthe promoted oxide solid is comprised of at least 1 wt. % MgO. Inanother embodiment, MgO promoter particles are present in the promotedoxide solid such that the promoted oxide solid is comprised of less thanor equal to about 25 wt. % MgO.

Within this disclosure, “substantial absence of CO and H₂” in thecombustion gases means that the combustion gases are comprised of lessthan 1.0 volume percent (vol %) CO and H₂.

Within this disclosure, “reducing” or “reduction” as it applies to ametal oxide particle means the loss of oxygen from the metal oxideparticle resulting in the formation of a reduced metal oxide particle.For example, the reduction of a generic Me_(x)O_(y) composition to ageneric Me_(z)O_(y-1) composition. Similarly, a “reduction product”means a chemical composition resulting from the reduction of a metaloxide particle.

Within this disclosure, “oxidizing” or “oxidation” as it applies to areduced metal oxide particle means the gain of oxygen by the reducedmetal oxide particle resulting in the formation of a metal oxideparticle. For example, the oxidation of a generic Me_(z)O_(y-1)composition to a generic Me_(x)O_(y) composition. Similarly, an“oxidation product” means a chemical composition resulting from theoxidation of reduced metal oxide particles.

Within this disclosure, “oxygen carrier” means a solid comprised of apromoted oxide solid, where the promoted oxide solid is comprised of aplurality of metal oxide particles and a plurality of MgO promoterparticles, where at least some portion of the plurality of metal oxideparticles is in physical contact with at least some portion of the MgOpromoter particles. Similarly, a “reduced carrier” means a solidcomprised of a plurality of reduced metal oxide particles and theplurality of MgO promoter particles, where the plurality of reducedmetal oxide particles are a reduction product of the plurality of metaloxide particles.

Within this disclosure, “regenerated metal oxide particles” means anoxidation product of a plurality of reduced metal oxide particles, and“regenerated promoted oxide solid” means a solid comprised of aplurality of regenerated metal oxide particles and a plurality of MgOpromoter particles. Similarly, a “regenerated oxygen carrier” means asolid comprised of the regenerated promoted oxide solid.

As discussed, the oxygen carrier is comprised of a promoted oxide solid,where the promoted oxide solid is comprised of a plurality of metaloxide particles and a plurality of MgO promoter particles, where theoxygen carrier exhibits a crystalline structure of both the metal oxideand MgO under XRD crystallography following the reduction-oxidationcycles described above. As an example, FIG. 2 indicates a series of XRDtraces for an embodiment where the metal oxide particle is Fe₂O₃, andfollowing 15 reduction-oxidation cycles using methane at approximately800° C. as the gaseous hydrocarbon. At FIG. 2, trace 211 represents anoxygen carrier comprised of 5 wt. % MgO, trace 212 represents an oxygencarrier comprised of 10 wt. % MgO, and trace 213 represents an oxygencarrier comprised of 25 wt. % MgO, where XRD peaks are labeled inaccordance with the accompanying symbol key. As indicated at traces 211,212, and 213, crystalline structures at the respective Fe₂O₃ and MgO2-theta (2Θ) values indicate the presence of both Fe₂O₃ and MgOfollowing the 15 cycles. For reference, FIG. 2 additionally illustratesan XRD trace of a Fe₂O₃ sample without the MgO promoter present at trace209, and an XRD trace of an MgO promoter comprised of MgO and withoutFe₂O₃ present at trace 210. Additionally, FIG. 2 indicates the formationof only a very small amount of MgFe₂O₄ under the reduction-oxidationconditions utilized, but a majority remains as MgO and Fe₂O₃.

The MgO promoter particles are an inert material with no significantoxygen release capability under the conditions of this disclosure.Without being bound by theory, it is believed that the MgO promoterparticles participate in the catalytic decomposition of the gaseoushydrocarbon and significantly improve the rate of combustion, therebyimproving the oxidation ability of the metal oxide particles comprisingthe oxygen carrier. This improvement is achieved by the catalyticdecomposition of methane producing carbon, carbon monoxide, andhydrogen, which oxidize over the metal oxide to generate CO₂ and H₂O.

As an example, when the metal oxide of the oxygen carrier is Fe₂O₃ andthe gaseous hydrocarbon is CH₄, the main reactions on the metal oxideFe₂O₃ when the oxygen carrier and the gaseous hydrocarbon are contactedat the temperatures of this disclosure are:

12Fe₂O₃+CH₄→CO₂+8Fe₃O₄+2H₂O

8Fe₂O₃+CH₄→CO₂+FeO+H₂O

However, in addition, the MgO promoter particles of the oxygen carrierprovides catalytic decomposition of the methane and promotes concurrentcarbon and hydrogen formation, which further oxidize over the Fe₂O₃:

CH₄→C+2H₂ (catalyzed by MgO)

2C+12Fe₂O₃→2CO₂+8Fe₃O₄

C+2Fe₂O₃→CO₂+4Fe₃O₄

4H₂+12Fe₂O₃→4H₂O+8Fe₃O₄

H₂+2Fe₂O₃ →H ₂O+4FeO

The presence of the MgO promoter particles in the promoted oxide solidof this disclosure significantly increases the oxygen transfer of themetal oxide and greatly improves the reaction rate and percent ofcombustion of the gaseous hydrocarbon, as compared to the metal oxidecarrier acting alone. The addition of an inert material such as MgO canalso increase the physical strength of the oxygen carrier. Thus, theaddition of MgO promoter particles increases the methane reactivity andphysical stability of the oxygen carrier and has a potential for useimproving industrial oxygen carriers for the chemical looping combustionprocess.

Proof of Principle

An oxygen carrier comprised of a promoted oxide solid containing Fe₂O₃and MgO was evaluated over fifteen cycle TGA combustion performance ofoxygen carriers prepared by the various preparation methods with 20%methane, balance He at 800° C. The addition of MgO-containing materialsto Fe₂O₃ increased the methane reactivity and physical stability of theoxygen carrier, and the presence of MgO indicated a strong improvementin combustion over a pure Fe₂O₃ species alone.

Oxygen Carrier Preparation:

Magnesium hydroxide (Mg(OH)₂) and Fe₂O₃ (Hematite) were commerciallyobtained from Sigma-Aldrich Co. LLC, St. Louis, Mo. Dolomite(CaMg(CO₃)₂) was commercially obtained from Alfa Aesar, Ward Hill, Mass.The oxygen carriers were prepared using several methods:

Physical mixing: Samples were prepared by physical mixing the solidsusing a mortar and pestle. MgO-containing materials with MgO weightloadings of 5, 10, and 25% were physically mixed with Fe₂O₃ and thencalcined at 850° C. in air for 3 hours. The calcined material was thencrushed to 35 mesh (500 micron) particle size.

Incipient wetness impregnation: A concentration of nitrates of magnesiumwere mixed and dissolved in deionized water for 5% MgO weight loading onthe Fe₂O₃. The solution was then added drop wise to the Fe₂O₃ andmaintained at 60° C. while stirring. The resulting paste was dried at60° C. for 2 hr and calcined at 850° C. in air for 3 hours. The calcinedsamples were then crushed to 35 mesh (500 micron) particle size.

Thermogravimetric Analysis (TGA) Test Method:

TGA was conducted in a thermogravimetric analyzer (TA Model 2050) toinvestigate the redox properties of the mixed oxygen carriers.Approximately 100 mg of sample was placed in a 5-mm-deep, 10-mm-diametercrucible. The oxide mixture was heated in a quartz-bowl to 800 or 900°C. at a heating rate of 10° C./min in N₂ gas and a flow rate of 100sccm. The sample was then maintained isothermal for the duration of theredox cycles. The reduction cycle consisted of 20% CH₄ balance nitrogenat 100 sccm, the oxidation cycle consists of air at 100 sccm. The samplewas purged in 100 sccm N₂ between each reduction and oxygen cycle toprevent mixing of methane and air.

The fractional reduction and oxidation was calculated using the TGA datadefined as: Fractional Reduction (X)=(M_(o)−M)/(M_(o)−M_(f)), where M isthe instantaneous weight of metal oxide, M_(o) is the initial weight ofthe metal oxide, and M_(f) is the weight of metal oxide followingreaction. The global rates of reactions (dX/dt) were calculated bydifferentiating the fractional conversions (X).

Lab-Scale Flow Reactor Test Method:

Fixed-Bed lab-scale reactor studies were carried out using aMicromeritics AutochemHP lab-scale quartz reactor. Approximately 1.0 gof sample was placed in the stainless steel reactor coupled with aPfeiffer Mass Spectrometer (MS). The reactor effluent was monitoredcontinuously for the MS responses corresponding to CH₄ (m/z=16), H₂O(m/z=18) and CO₂ (m/z—44). The inlet flow of 100% He was maintained at atotal flow rate of 60 cm³/min over the oxygen carriers at 101.3 kPawhile heating from 25 to 800° C. at a heating rate of 10° C./min. Thesample was then maintained isothermal for the duration of the redoxcycles. The reduction cycle consisted of 20% CH₄ balance He at 60 sccm,while the oxidation cycle consisted of air at 60 sccm. The sample waspurged in 60 sccm He between each reduction and oxygen cycle to preventmixing of methane and air.

Results of the Fifteen Cycle TGA Tests for the CLC Reaction on 5%, 10%,and 25% Dolomite/MgO—Fe₂O₃ (hematite) Oxygen Carriers:

Fifteen cycle TGA combustion performance of oxygen carriers prepared bythe various preparation methods with 20% methane are summarized inTable 1. Data for cycles 2, 8, and 15 are reported. The oxygen carrierpreparation method is indicated as applicable by Note (a) and Note (b).Percent combustion (% Comb.) was based on the moles of CH₄ introduced tothe lab scale reactor compared to the moles of CO₂ produced, and theglobal reaction rate (Rate) indicates the rate of conversion of molesCH₄ to moles CO₂ per minute.

Pure unsupported Fe₂O₃ showed a percentage combustion of 15.0% on cycle2 and a global reaction rate of 0.31 min⁻¹, and the global reaction rateremained fairly constant during the 15 reduction cycles. The combustionpercentage of Fe₂O₃ from 15.0% (cycle 2) to 19.9% (cycle 15) maypossibly be the result of the changing surface area of the oxygencarrier. The MgO and dolomite, though theoretically un-reactive for theCLC combustion reaction, show very low (approximately 6%) combustion ofmethane.

The addition of 5% MgO to Fe₂O₃ by the incipient wetness impregnationmethod increased the combustion percentage of Fe₂O₃ from 15.0% to 35.4%and the global reaction rate from 0.31 min⁻¹ to1.43 min⁻¹ on cycle 2.The combustion percentage (35.4-40.0%) and global rate (1.43-1.34 min⁻¹)remained relatively constant through the 15 cycle tests. For the samplesprepared by physical mixing of the 5% wt Mg(OH)₂ and Fe₂O₃ thecombustion percentage (28.9%) was similar to that of the carrierprepared by the impregnation method (35.4) but there was a slightincrease in reaction rate from 1.43 min⁻¹ to 1.69 min⁻¹.

Increasing the amount of MgO loading on the Fe₂O₃ (cycle 2 data)resulted in an increase in combustion percentage from 35.4% (5% loading)to 40.6% (25% loading). The overall trend in Table 1 shows thatincreasing the amount of MgO-containing material increases the % oxygentransport capacity significantly. The addition of the MgO-containingmaterials result in significantly higher global reaction rates than thatof the pure unsupported Fe₂O₃ oxygen carrier even after 15 redox cycles.

The raw dolomite material contains 54.4% CaCO₃ and 45.4% MgCO₃.Calcination of the dolomite material at 850° C. removes the CO₂component of dolomite producing 54.4% CaO and 45.6% MgO which was thenmixed at 5, 10, and 25 wt % with Hematite. According to Table 1, The TGAdata of the pure calcined dolomite indicates that dolomite alone isun-reactive for CLC combustion. Addition of 5 wt % dolomite to Fe₂O₃(hematite) caused an increase in combustion percentage from 15.0% (pureFe₂O₃) to 31.6% and increased the global reaction rate from 0.31 to 1.37min⁻¹. Similar to the MgO/Fe₂O₃ oxygen carriers, the dolomite/Fe₂O₃oxygen carrier exhibited a significant increase in the global reactionrate by the addition of dolomite and the rate remained relatively stableon increasing number of reduction/oxidation cycles. The combustionpercentage also remained relatively stable during the cyclic tests.Examining Table 1, a global trend appears over all of the MgO/Fe₂O₃oxygen carriers: The combustion percentage increases on an increasingnumber of reduction cycles similar to that of the pure Fe₂O₃ material.This is consistent with the inert materials promoting carbon formationand the oxidation to CO₂ occurring by the reaction of carbon and Fe₂O₃oxygen carrier.

Results of Fifteen Cycle Tests in the Lab Scale Reactor.

FIG. 3 illustrates moles CO₂ produced per 100 grams of oxygen carrier(moles CO₂/100-g) over 15 cycles at 800° C. for dolomite/Fe₂O₃ oxygencarriers prepared by physical mixing. At FIG. 3, curve 315 representsthe performance of pure unsupported Fe₂O₃, and curve 314 represents theperformance of 100% dolomite. Curve 316 represents the performance of anoxygen carrier comprised of 5% dolomite, balance hematite, curve 317represents the performance of an oxygen carrier comprised of 10%dolomite, balance hematite, and curve 318 represents the performance ofan oxygen carrier comprised of 25% dolomite, balance hematite. Asillustrated, the addition of MgO comprising the dolomite to the Fe₂O₃significantly increases the CO₂ production, and the reaction performanceis stable for methane combustion. Further, no agglomeration of theoxygen carriers was observed after the 15 cycle tests. Additionally, thesurface area of the pure unsupported Fe₂O₃ and the oxygen carriers waswithin the range 0.1551-0.2451 m²/g, indicating that the enhancementillustrated at FIG. 3 is not due to a surface area effect.

FIG. 4 illustrates moles CO₂/100-g over 15 cycles at 800° C. forMgO/Fe₂O₃ oxygen carriers prepared by incipient wetness impregnation.Curve 415 represents the performance of pure unsupported Fe₂O₃, andcurve 414 represents the performance of 100% MgO. Curve 416 representsthe performance of an oxygen carrier comprised of 5% MgO (nitrate),balance hematite, curve 417 represents the performance of an oxygencarrier comprised of 10% MgO (nitrate), balance hematite, and curve 418represents the performance of an oxygen carrier comprised of 25% MgO(nitrate), balance hematite. The data clearly shows the performanceenhancement due to MgO. The surface area of the pure unsupported Fe₂O₃and the oxygen carriers was within the range 0.1551-0.3933 m₂/g,indicating that the enhancement illustrated at FIG. 3 is not due to asurface area effect.

FIG. 5 illustrates a % Oxygen used, based on the moles of oxygen in theCO₂ produced divided by the theoretical amount of O₂ available in theFe₂O₃ present. Curve 519 represents the % Oxygen used of pureunsupported Fe₂O₃, while curve 520 represents the % Oxygen used of theoxygen carrier comprised of 5% dolomite, balance hematite prepared byphysical mixing, and curve 521 represents the % Oxygen of the oxygencarrier comprised of 5% MgO (nitrate), balance hematite prepared byincipient wetness impregnation. FIG. 5 indicates that MgO significantlyimproves the combustion efficiency of the Fe₂O₃, and indicates that thereaction performance enhancing effect of the MgO-containing materialshas a potential for promoting industrial oxygen carriers for use in thechemical looping combustion process.

FIG. 6 illustrates the results of fifteen cycle TGA combustionperformance of an oxygen carrier comprised of 5% MgO, balance Fe₂O₃,compared to pure, unsupported Fe₂O₃ at several temperatures, andindicates the impact of temperature on the reaction performanceenhancing effect. Curves 622 and 623 correspond to a redox cycletemperature of 700° C., where curve 622 indicates moles CO₂/100-ggenerated over pure, unsupported Fe₂O₃ while curve 623 indicates molesCO₂/100-g generated over a 5% MgO, balance Fe₂O₃ oxygen carrier.Similarly, curves 624 and 625 correspond to a redox cycle temperature of800° C., where curve 624 indicates moles CO₂/100-g generated over pure,unsupported Fe₂O₃ while curve 625 indicates moles CO₂/100-g generatedover a 5% MgO, balance Fe₂O₃ oxygen carrier. Additionally, Curves 626and 627 correspond to a redox cycle temperature of 900° C., where curve626 indicates moles CO₂/100-g generated over pure, unsupported Fe₂O₃while curve 627 indicates moles CO₂/100-g generated over a 5% MgO,balance Fe₂O₃ oxygen carrier. As indicated by FIG. 6, available oxygenincreases on increasing temperature, with a low temperature limitgenerally around 725° C. At 900 ° C. MgO contributes to the bestperformance, and reduction of Fe₂O₃ to Fe was observed. However,formation of Fe metal may have contributed to agglomeration as indicatedby a decrease in capacity during fifteen cycles. Better design of theoxygen carrier may be necessary to avoid agglomeration at 900 ° C. tomaintain the high performance.

Thus, the disclosure herein provides an oxygen carrier comprised of apromoted oxide solid, where the promoted oxide solid is comprised of aplurality of metal oxide particles in contact with a plurality of MgOpromoter particles. The MgO promoter particles increase the reactionrate and oxygen utilization of the metal oxide when contacting with agaseous hydrocarbon at a temperature greater than about 725° C. Thepromoted oxide solid is generally comprised of between 5 wt. % and 25wt. % MgO, and may be prepared by physical mixing, incipient wetnessimpregnation, or other methods known in the art for generating aplurality of metal oxide particles in contact with a plurality of MgOparticles. The oxygen carrier exhibits a crystalline structure of themetal oxide and a crystalline structure of MgO under XRDcrystallography, and retains these crystalline structures oversubsequent redox cycles. In an embodiment, the metal oxide is Fe₂O₃ andthe gaseous hydrocarbon is comprised of methane.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention and it is not intended to be exhaustive or limit the inventionto the precise form disclosed. Numerous modifications and alternativearrangements may be devised by those skilled in the art in light of theabove teachings without departing from the spirit and scope of thepresent invention. It is intended that the scope of the invention bedefined by the claims appended hereto.

In addition, the previously described versions of the present inventionhave many advantages, including but not limited to those describedabove. However, the invention does not require that all advantages andaspects be incorporated into every embodiment of the present invention.

All publications and patent documents cited in this application areincorporated by reference in their entirety for all purposes to the sameextent as if each individual publication or patent document were soindividually denoted.

TABLE 1 Combustion Reaction performance with the addition of MgO anddolomite to Fe₂O₃ Cycle 2 Cycle 8 Cycle 15 Oxygen Carrier % Comb. Rate(min⁻¹) % Comb. Rate (min⁻¹) % Comb. Rate (min⁻¹) Fe₂O₃ 15.0 0.31 18.70.32 19.9 0.32 MgO 1.8 — 1.0 — 0.9 — Dolomite 5.8 — 6.0 — 6.1 —  5%MgO/Fe₂O₃ (a) 35.4 1.43 38.9 1.39 40 1.34  5% MgO/Fe₂O₃ (b) 28.9 1.6928.9 1.07 30.4 0.98 10% MgO/Fe₂O₃ (b) 30.3 1.29 28.9 1.15 32.7 1.05 25%MgO/Fe₂O₃ (b) 40.6 1.31 39.5 1.18 39.9 1.07  5% Dolomite/Fe₂O₃ (b) 31.61.37 32.1 1.57 37 1.48 10% Dolomite/Fe₂O₃ (b) 30.3 1.62 28.9 1.3  32.71.19 25% Dolomite/Fe₂O₃ (b) 23 1.65 26.8 1.21 29.7 1.13 Note (a)Incipient wetness impregnation preparation Note (b) Physical mixingpreparation

What is claimed is:
 1. A method of combusting a gaseous hydrocarboncomprising: establishing the gaseous hydrocarbon at a temperaturegreater than 725° C., thereby generating a heated gaseous hydrocarbon;and contacting the heated gaseous hydrocarbon and an oxygen carrier,where the oxygen carrier is comprised of a promoted oxide solid, wherethe promoted oxide solid is comprised of a plurality of metal oxideparticles and a plurality of MgO promoter particles, and where at leastsome portion of the plurality of metal oxide particles is in physicalcontact with at least some portion of the MgO promoter particles,thereby producing combustion gases and a reduced carrier, where thecombustion gases are comprised of CO₂ and H₂O and where the combustiongases have a substantial absence of CO and H₂, and where the reducedcarrier is comprised of a plurality of reduced metal oxide particles andthe plurality of MgO promoter particles, where the plurality of reducedmetal oxide particles are a reduction product of the plurality of metaloxide particles, thereby combusting the gaseous hydrocarbon;
 2. Themethod of claim 1 further comprised of: separating the reduced carrierand the heated gaseous hydrocarbon and the combustion gases; andexposing the reduced carrier to an oxidizing gas comprised of O₂,thereby oxidizing a portion of the plurality of reduced metal oxideparticles, and thereby producing a plurality of regenerated metal oxideparticles, where the plurality of regenerated metal oxide particles isan oxidation product of the plurality of reduced metal oxide particles,and thereby producing a regenerated promoted oxide solid, where theregenerated promoted oxide solid is comprised of the plurality ofregenerated metal oxide particles and the plurality of MgO promoterparticles, and thereby producing a regenerated oxygen carrier, where theregenerated oxygen carrier is comprised of the regenerated promotedoxide solid.
 3. The method of claim 1 where the gaseous hydrocarbon iscomprised of methane.
 4. The method of claim 1 where the plurality ofmetal oxide particles comprise at least 50 weight percent of thepromoted oxide solid.
 5. The method of claim 4 where the plurality ofMgO promoter particles are present in the promoted oxide solid such thatthe promoted oxide solid is comprised of less than or equal to about 25weight percent MgO.
 6. The method of claim 1 where the oxygen carrier isfurther comprised of a plurality of promoted oxide solids in contactwith a binder.
 7. A method of combusting the gaseous hydrogen using themethod of claim 2 further comprised of: maintaining a fuel reactor at atemperature greater than about 725° C., and flowing a gaseous streaminto the fuel reactor, where the gaseous stream is comprised of thegaseous hydrocarbon, thereby establishing the gaseous hydrocarbon at atemperature greater than about 725° C. and thereby generating a heatedgaseous hydrocarbon; delivering the oxygen carrier to the fuel reactor;mixing the heated gaseous hydrocarbon and the oxygen carrier in the fuelreactor, thereby contacting the heated gaseous hydrocarbon and theoxygen carrier, and thereby producing the combustion gases and thereduced carrier; withdrawing the combustion gases from the fuel reactor;transferring the reduced carrier to a reducing reactor, therebyseparating the reduced carrier and the heated gaseous hydrocarbon andthe combustion gases; imparting the oxidizing gas to the reducingreactor and mixing the oxidizing gas and the reduced carrier, therebyexposing the reduced carrier to the oxidizing gas, and thereby producingthe plurality of regenerated metal oxide particles, and therebyproducing the regenerated promoted oxide solid, and thereby producingthe regenerated oxygen carrier; transporting the regenerated oxygencarrier to the fuel reactor and repeating the mixing, withdrawing,transferring, and imparting steps using the regenerated oxygen carrieras the oxygen carrier.
 8. The method of claim 2 where the oxygen carrierexhibits a crystalline structure of the metal oxide and a crystallinestructure of MgO under XRD crystallography.
 9. The method of claim 8where the metal oxide is Fe₂O₃, CuO, NiO, Mn₂O₃, Co₃O₄, CaSO₄, MoO ormixtures thereof.
 10. A method of combusting a gaseous hydrocarboncomprising: maintaining a fuel reactor at a temperature greater thanabout 725° C.; flowing a gaseous stream into the fuel reactor, where thegaseous stream is comprised of the gaseous hydrocarbon, and heating thegaseous hydrocarbon to a temperature greater than about 725° C., therebygenerating a heated gaseous hydrocarbon; delivering an oxygen carrierinto the fuel reactor, where the oxygen carrier is comprised of apromoted oxide solid, where the promoted oxide solid is comprised of aplurality of metal oxide particles and a plurality of MgO promoterparticles, and where at least some portion of the plurality of metaloxide particles is in physical contact with at least some portion of theMgO promoter particles; contacting the heated gaseous hydrocarbon andthe oxygen carrier in the fuel reactor, thereby producing combustiongases and a reduced carrier, where the combustion gases are comprised ofCO₂ and H₂O and where the combustion gases have a substantial absence ofCO and H₂, and where the reduced carrier is comprised of a plurality ofreduced metal oxide particles and the plurality of MgO promoterparticles, where the plurality of reduced metal oxide particles are areduction product of the plurality of metal oxide particles; withdrawingthe combustion gases from the fuel reactor; transferring the reducedcarrier to a reducing reactor; imparting an oxidizing gas comprised ofO₂ to the reducing reactor and exposing the reduced carrier to theoxidizing gas, thereby oxidizing a portion of the plurality of reducedmetal oxide particles, and thereby producing a plurality of regeneratedmetal oxide particles, where the plurality of regenerated metal oxideparticles is an oxidation product of the plurality of reduced metaloxide particles, and thereby producing a regenerated promoted oxidesolid, where the regenerated promoted oxide solid is comprised of theplurality of regenerated metal oxide particles and the plurality of MgOpromoter particles, and thereby producing a regenerated oxygen carrier,where the regenerated oxygen carrier is comprised of the regeneratedpromoted oxide solid; and transporting the regenerated oxygen carrier tothe fuel reactor and repeating the contacting, withdrawing, andimparting steps using the regenerated oxygen carrier as the oxygencarrier.
 11. The method of claim 10 where the plurality of metal oxideparticles comprise at least 50 weight percent of the promoted oxidesolid.
 12. The method of claim 11 where the plurality of MgO promoterparticles are present in the promoted oxide solid such that the promotedoxide solid is comprised of less than or equal to about 25 weightpercent MgO.
 13. The method of claim 12 where the oxygen carrierexhibits a crystalline structure of the metal oxide and a crystallinestructure of MgO under XRD crystallography.
 14. The method of claim 13where the metal oxide is Fe₂O₃, CuO, NiO, Mn₂O₃, Co₃O₄, CaSO₄, MoO ormixtures thereof
 15. The method of claim 14 where the oxidizing gas isair.
 16. The method of claim 15 where the gaseous hydrocarbon iscomprised of methane.
 17. The method of claim 15 where the metal oxideis Fe₂O₃.
 18. A method of combusting a gaseous hydrocarbon comprising:maintaining a fuel reactor at a temperature greater than about 725° C.;flowing a gaseous stream into the fuel reactor, where the gaseous streamis comprised of the gaseous hydrocarbon, and heating the gaseoushydrocarbon to a temperature greater than about 725° C., therebygenerating a heated gaseous hydrocarbon; delivering an oxygen carrierinto the fuel reactor, where the oxygen carrier is comprised of apromoted oxide solid, where the promoted oxide solid is comprised of aplurality of Fe₂O₃ particles and a plurality of MgO promoter particles,and where at least some portion of the plurality of Fe₂O₃ particles isin physical contact with at least some portion of the MgO promoterparticles, and where the plurality of MgO promoter particles are presentin the promoted oxide solid such that the promoted oxide solid iscomprised of less than or equal to about 25 weight percent MgO;contacting the heated gaseous hydrocarbon and the oxygen carrier in thefuel reactor, thereby producing combustion gases and a reduced carrier,where the combustion gases are comprised of CO₂ and H₂O and where thecombustion gases have a substantial absence of CO and H₂, and where thereduced carrier is comprised of a plurality of reduced metal oxideparticles and the plurality of MgO promoter particles, where theplurality of reduced metal oxide particles are a reduction product ofthe plurality of Fe₂O₃ particles; withdrawing the combustion gases fromthe fuel reactor; transferring the reduced carrier to a reducingreactor; imparting an oxidizing gas comprised of O₂ to the reducingreactor and exposing the reduced carrier to the oxidizing gas, therebyoxidizing a portion of the plurality of reduced metal oxide particles,and thereby producing a plurality of regenerated metal oxide particles,where the plurality of regenerated metal oxide particles is an oxidationproduct of the plurality of reduced metal oxide particles, and therebyproducing a regenerated promoted oxide solid, where the regeneratedpromoted oxide solid is comprised of the plurality of regenerated metaloxide particles and the plurality of MgO promoter particles, and therebyproducing a regenerated oxygen carrier, where the regenerated oxygencarrier is comprised of the regenerated promoted oxide solid; andtransporting the regenerated oxygen carrier to the fuel reactor andrepeating the contacting, withdrawing, and imparting steps using theregenerated oxygen carrier as the oxygen carrier.
 19. The method ofclaim 18 where the oxygen carrier exhibits a crystalline structure ofFe₂O₃ and a crystalline structure of MgO under XRD crystallography 20.The method of claim 19 where the oxidizing gas is air.